With the recent trend of electronic devices toward higher performance, circuit boards have been required to be thinner, and electronic components have been required to be mounted more densely. To meet such requirements, electronic components such as chip components, CSPs (chip size packages), and ICs have been designed to be smaller and provide higher performance, and circuit boards have been provided with narrow pitch wiring to achieve micro wiring. This has lead to higher packing density of electronic components and rising prices per electronic component. Further, due to increased added value per circuit board with electronic components mounted thereon, prices per circuit board also tend to rise.
When an electronic component is mounted on a circuit board, a thermosetting adhesive is dispensed to a predetermined position of the circuit board, and the electronic component is placed at the predetermined position. Heat is then applied to cure the thermosetting adhesive, so that the component is temporarily fixed to the predetermined position of the circuit board. Flux is then applied to the area of the circuit board to be connected with the electronic component, and the circuit board is immersed in molten solder. As a result, the electronic component is electrically connected to the circuit board to obtain a mounting structure.
Dispensing methods of surface mount adhesives include, for example, the air method and the screw method. The air method utilizes air pressure to feed a surface mount adhesive from a syringe to a nozzle. Since the air method utilizes only air pressure to control the amount of a surface mount adhesive applied, the applied amount tends to vary and become unstable with a decrease of the amount of the surface mount adhesive inside the syringe.
The screw method utilizes air pressure and screw rotation to dispense a surface mount adhesive to a predetermined position. Since the screw method utilizes air pressure and a screw to control the applied amount, the applied amount becomes more stable. Hence, when a surface mount adhesive is dispensed with the application area diameter set to, for example, approximately 0.6 mm, the screw method is commonly used as in JP A No. 2001-135927.
WO 99/60622 proposes an insulating adhesive for a semiconductor chip, wherein a resin into which a filler is to be dispersed has a fixed mean molecular weight or viscosity.
The surface mount adhesives of JP A No. 2001-135927 and WO 99/60622 contain a filler. Since the dispensing of a surface mount adhesive is conducted at a high speed cycle of approximately 0.07 s/shot, the following dispensing failures may occur unless the physical properties of the filler are optimized.
First, when a nozzle is brought into contact with a circuit board to dispense a surface mount adhesive to the circuit board, the nozzle is pressed against the circuit board, and thus part of the surface mount adhesive being dispensed may splash due to pressure.
Second, if the surface mount adhesive is not smoothly dropped on the circuit board, the surface mount adhesive may splash or tail (string) when the nozzle moves from one position to another to dispense the surface mount adhesive.
Third, if the wettability of the surface mount adhesive relative to the nozzle is insufficient, the surface mount adhesive may adhere or stick to the nozzle if it is continuously dispensed. This may cause the applied amount to become unstable or cause a nozzle clogging.